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<br />7A - ATTACHMENT NO.3 <br />Saltworks Proposa/- Water Group Summary Report (22 January 2010) Page 41 <br /> <br />of shallow and deep monitoring wells to assess groundwater quality issues, determine <br />water level fluctuations and the hydraulic connections (or lack thereof) between the <br />shallow and deep aquifers, evaluate site-specific aquifer parameters, conduct basin-wide <br />and local water balance studies factoring in reverse osmosis and other issues, and <br />better understand the hydrogeologic framework of the site. <br /> <br />3.5.2.3 Subsidence <br /> <br />Land subsidence is the lowering of the natural ground surface, which can result in <br />damage to structures, disrupt the gradient of streams and flood control channels, and <br />result in increased flooding potential for relatively low-elevation areas like the project <br />site. Land subsidence can occur as a result of significant lowering of the water table due <br />to intensive pumping from a confined aquifer (Borchers, 1998). Subsidence results from <br />loss of hydrostatic pressure and subsequent compaction of clay layers, which are highly <br />compressible. The amount of compaction is a function of thickness and vertical <br />permeability of the clay, time and magnitude of the groundwater level decline, and the <br />microstructure of the clay. <br /> <br />Subsidence occurs in groundwater basins with a significant thickness of saturated, <br />unconsolidated, fine-grained (clayey) sediments, such as portions of the Santa Clara <br />Valley. In the Santa Clara Valley, subsidence as much as 6 feet was recorded over 50 <br />years (1915-1965) and ceased when groundwater levels recovered. Once subsidence <br />occurs, it is permanent and produces a permanent loss of groundwater storage. As one <br />example, the bayside community of Alviso subsided below sea level necessitating <br />construction of levees to prevent flooding of the community. The Santa Clara Valley <br />Water District now manages the groundwater basin to maintain groundwater elevations <br />above the level at which subsidence last occurred (SCVWD, 2000). <br /> <br />Regional drawdowns in wells in excess of 100 feet were common throughout the area of <br />subsidence and contributed to the problem. Although the regional subsidence discussed <br />above has not been significant in the Redwood City area, the lack of pumping wells may <br />be the reason. Reducing the available drawdown in future wells seems prudent and will <br />also reduce the regional depth of the cone of depression, which will minimize the risk of <br />increasing regional subsidence. Local subsidence, rather than regional, in the near- <br />vicinity (within 2 feet) of a production well can also produce structural failures of the well <br />itself. If a groundwater resource is developed at the Saltworks site, additional mitigating <br />measures for subsidence may need to be implemented including a reduction of <br />recommended drawdowns in wells. As previously noted, well yields were estimated for <br />100 feet of drawdown. If drawdown needs to be limited to 50 feet, well yields would be <br />reduced to about 50 to 60 gpm, increasing the number of wells required. <br /> <br />The issue of potential land subsidence is addressed by EKI (2006), which acknowledges <br />potential subsidence and recommends evaluation by geotechnical consultants and a <br />program of subsidence monitoring. The project probably will not result in regional <br />subsidence but more likely will produce local subsidence within the extent of the cone of <br />depression established by production wells. Observations of structurally failed wells in <br />the 1940s by Poland and Garrett (1943) suggest that local subsidence could be an <br />issue. EKI (2009) states that measurable subsidence in the future could warrant <br />substantial curtailment or cessation of pumping. The water team concurs with this <br />conclusion. <br /> <br />34 <br />